1. Field of the Invention
The present invention relates generally to an induction motor controller and particularly to a controller which allows an accurate estimate of the rotary angular velocity of an induction motor when a primary resistance value or a secondary resistance value of the induction motor is varied due to temperature, using vector control, without a velocity detector.
2. Description of the Background Art
FIG. 9 is a block diagram illustrating an induction motor controller disclosed in the Japanese Patent Disclosure Bulletin No. 1989-043097. In that controller, a three-phase induction motor 1 is connected to receive the output from power amplifier 2 operating under the control of a coordinate converter 3. Coordinate converter 3 converts primary voltage command values v.sub.d e*s and v.sub.q e*s (hereinafter, subscripts d.sup.e* s and q.sup.e* s represent primary components along d.sup.e and q.sup.e axes) along orthogonal coordinate axes (d.sup.e -q.sup.e axes) rotating at a primary circular frequency .omega.. The power amplifier 2 and the coordinate converter 3 constitute a power feeder. A second coordinate converter 4 operates to convert the three-phase alternating current at the input (i.e., primary currents, i.sub.u, i.sub.v, i.sub.w) into an exciting current component, i.sub.d e.sub.s, and a torque current component, i.sub.q e.sub.s, primary currents along the d.sup.e -q.sup.3 axes. A third coordinate converter 5 is operative to convert the three-phase alternating-current voltages at the motor input (i.e., primary voltages, v.sub.u, v.sub.v, v.sub.w) into primary voltages, v.sub.d e.sub.s and v.sub.q e.sub.s, along the d.sup.e -q.sup.e axes. An equivalent-flux generator 6 operates to generate secondary linkage magnetic flux (SLMF) equivalent values .lambda.'d.sup.e r, .lambda.'q.sup.e r (hereinafter, subscripts d.sup.e r and q.sup.e r represent secondary components along the d.sup.e and q.sup.e axes) in response to the primary voltages. A magnetic flux/slip frequency estimator 7 operates in response to the exciting current component and the torque current component to generate an estimated value, .lambda.d.sup.e r, of the secondary linkage magnetic flux and an estimated value, p.omega..sub.s, of the slip frequency in a vector-controlled state. A flux estimating device 8 generates estimated values .lambda.'d.sup.e r and .lambda.'q.sup.e r of the secondary linkage magnetic flux equivalent amounts. A rotary velocity estimator 9 operates in response to the estimated values from flux estimating device 8 and the equivalent flux generator 6 to generate an estimated value, p.omega..sub.r, of the rotary angular velocity of the induction motor 1. The value .omega. is formed at adder 12 by adding p.omega..sub.r and p.omega..sub.s and is input to an integrator 10, whose output .theta. is provided to a trigonometric function generator 11 whose sine and cosine outputs are provided to converter 5. An adder 12, subtracters 13 and 14, and PI (proportional integral) compensators 15 and 16 complete the circuit.
In operation, the equivalent flux generator 6 receives the primary voltages, v.sub.d e.sub.s, v.sub.q e.sub.s, the primary currents, i.sub.d e.sub.s, i.sub.q e.sub.s, and the primary circular frequency, .omega., and generates secondary linkage magnetic flux equivalent amounts, .lambda.'d.sup.e r and .lambda.'q.sup.e r, according to matrix expression (1): ##EQU1## where Rs, Ls, Lr, M and .sigma. are fixed values of the primary resistance, primary inductance, secondary inductance, mutual inductance and the leakage coefficient of the induction motor 1, respectively. Also, P is a differential operator, and T is a time constant of first order lag.
The magnetic flux/slip frequency estimator 7 receives the primary currents, i.sub.d e.sub.s, i.sub.q e.sub.s, and generates an estimated value, .lambda.d.sup.e r of the secondary linkage magnetic flux (SLMF) and the estimated value, p.omega..sub.s, of the slip frequency in the vector-controlled state according to expressions (2) and (3): ##EQU2## where, Rr is the fixed value of the secondary resistance of the induction motor 1.
The flux estimating device 8 receives the estimated value, .lambda.d.sup.e r, of the secondary linkage magnetic flux and the primary circular frequency, .omega., and generates the estimated values, .lambda.'d.sup.e r and .lambda.'q.sup.e r, of the secondary linkage magnetic flux equivalent amounts according to matrix expression (4): ##EQU3##
The rotary angular velocity estimator 9 receives the secondary linkage magnetic flux equivalent amounts, .lambda.'d.sup.e r, .lambda.'q.sup.e r, the estimated values .lambda.'d.sup.e r, .lambda.'q.sup.e r, of the secondary linkage magnetic flux equivalent amounts, a fixed secondary resistance value Rr and the slip frequency estimated value p.omega..sub.s, and determines the estimated value, p.omega..sub.r, of the rotary angular velocity of the induction motor 1 according to expression (5): ##EQU4## where K is a positive constant.
The adder 12 adds the estimated value, p.omega..sub.r, of the rotary angular velocity and the estimated value, p.omega..sub.s, of the slip circular frequency and outputs the primary circular frequency, .omega.. The integrator 10 integrates the primary circular frequency, .omega., and outputs a phase signal, .theta.. The trigonometric function generator 11 inputs the phase signal .theta., and outputs a corresponding sine .theta. and a cosine .theta. value.
The subtractor 13 and the PI compensator 15 perform feedback control so that the exciting current component, i.sub.d e.sub.s, may follow up a command value, i.sub.d e.sub.s *. The subtractor 14 and the PI compensator 16 perform feedback control so that the torque component current, i.sub.q e.sub.s, may follow up a command value, i.sub.q e.sub.s *.
However, when the actual induction motor is controlled by this induction motor controller, temperatures of the primary resistance value, Rs (where subscript s refers to the stator as the source of resistance), and the secondary resistance value, Rr (where the subscript r refers to the rotor as the source of resistance), of the induction motor vary from 0.degree. C. to about 120.degree. C. in accordance with load state, ambient temperature and other conditions. Assuming that the resistance values center at 60.degree. C., for example, they will vary approximately 20% higher and lower. By omitting the differential term in the steady state, the operational expression of the secondary linkage magnetic flux equivalent amounts, .lambda.'d.sup.e r, .lambda.'q.sup.e r, represented by the expression (1) converts into expression (6): ##EQU5##
By ignoring the differential term, the estimated value, p.omega..sub.s, of the slip circular frequency in the steady state, according to expressions (2) and (3), converts into expression (7): ##EQU6##
Since expression (6) includes a fixed primary resistance value Rr, and expression (7) includes a fixed secondary resistance value, Rr, if an error occurs due to temperature variations between the fixed primary and secondary resistance values, Rs, Rr, and the actual primary and secondary resistance values, Rs, Rr, of the induction motor 1, the operational accuracy of the secondary linkage magnetic flux equivalents .lambda.'d.sup.e r, .lambda.'q.sup.e r, and the estimated value, p.omega..sub.s, of the slip frequency is lowered. Further, in expression (6), since .vertline..omega..sigma.Ls.vertline. becomes less than the fixed primary resistance value, Rs, in a low velocity range, the error in the primary resistance value, Rs, has greater influence on the secondary linkage magnetic flux equivalent amounts, .lambda.'d.sup.e r, .lambda.'q.sup.e r. Therefore, since the rotary angular velocity estimator 9 calculates its values using expression (5), which includes the secondary linkage magnetic flux equivalent amounts, .lambda.'d.sup.e r, .lambda.'q.sup.e r, the estimated value p.omega..sub.s of the slip frequency and the fixed secondary resistance value Rr, an estimation error, especially large in the low velocity range, occurs in the estimated value p.omega..sub.r of rotary angular velocity. The error is due to the variations of the primary resistance value Rs and the secondary resistance value Rr according to temperature, resulting in unstable control.
It is, accordingly, an object of the present invention to overcome the disadvantages in the above system by providing an induction motor controller that will allow an accurate estimation of the rotary angular velocity of the induction motor regardless of any variation in the primary resistance value or the secondary resistance value of the induction motor with temperature.
It is a further object of the present invention to provide an induction motor controller that will allow an accurate estimation of rotary angular velocity at low motor speeds.